1. Field of the Invention
The present invention relates to a process for producing an optical element such as a circularly-polarized-light-separating element or a color filter, and particularly to a method of producing an optical element by patterning liquid crystal films.
2. Description of Related Art
There has conventionally been known a process for producing an optical element such as a circularly-polarized-light-separating element or a color filter by the use of liquid crystal films having cholesteric regularity (cholesteric films). In such a process for producing an optical element, it is necessary to form a pattern whose outline coincides with the external shape of the optical element, or to form red-, green- and blue-colored patterns. There is, therefore, a demand for a method of patterning cholesteric liquid crystal films with high precision and efficiency.
Conventionally known methods of forming red-, green- and blue- colored patterns are as follows: (1) a method utilizing temperature changes to control the colors of cholesteric liquid crystal films, in which when a cholesteric liquid crystal film has turned the desired color, ultraviolet light is applied, through a mask, to a part of the cholesteric liquid crystal film so as to cure it in a desired pattern (see Japanese Laid-Open Patent Publications No. 258623/2000, No. 304613/1997 and No. 320480/1996); (2) a method in which liquid crystals of red, green and blue are embedded in grooves that have been provided in a predetermined pattern on a substrate (see Japanese Laid-Open Patent Publications No. 337716/1999 and No. 282324/1998); and (3) a method in which cholesteric liquid crystal films of different colors, patterned as desired, are successively transferred to one substrate (see Japanese Laid-Open Patent Publications No. 4824/2001 and No. 4822/2001).
The above-described method (1) utilizes temperature changes to control colors, so that this method has such problems that patterning precision is low and that the patterned cholesteric liquid crystal films of different colors show extremely poor optical properties at the boundaries between them. Moreover, although it is possible, by this method, to form red-, green- and blue-colored patterns on one cholesteric liquid crystal film, it is impossible to form the colored patterns by removing a part of a cholesteric liquid crystal film. The method (1) is thus disadvantageous in that a cholesteric liquid crystal film cannot be patterned into a shape whose outline coincides with the external shape of an optical element.
In the above-described method (2), liquid crystals are embedded in grooves that have been provided in a predetermined pattern on a substrate. Problems with this method are, therefore, such that colored patterns cannot be obtained with high positional precision and thickness precision and that many hours are required for production.
The above-described method (3) requires complicated production processes. Moreover, the patterned cholesteric liquid crystal films obtained by this method are poor in optical properties, heat resistance, chemical resistance, etc.
In addition to the aforementioned methods (1), (2) and (3), there has been proposed a method in which ultraviolet light is applied, through a mask, to cholesteric liquid crystal films of red, green and blue to form red-, green- and blue-colored patterns (see Japanese Laid-Open Patent Publication No. 153789/1999). In this method, it is necessary to remove the uncured portion of the cholesteric liquid crystal films. Usually used for this purpose is a technique of physically separating the uncured portion of a cholesteric liquid crystal film. One problem with this technique is that high thickness precision cannot be attained at the boundaries between patterns of different colors. In addition, the surface of the substrate having the function of aligning liquid crystalline molecules tends to be damaged, so that when a liquid crystal is applied again to this surface, liquid crystalline molecules in this liquid crystal are not easily aligned.
In the connection with this method, known is a method of removing the uncured portion of the cholesteric liquid crystal film by the use of an organic solvent (see Japanese Laid-Open Patent Publication No. 74924/1990). However, this method is merely meant for forming a single liquid crystal film on a substrate; and there is no consideration for how the organic solvent affects the aligning function of the substrate or the liquid crystal film.